The invention concerns providing wave dispersive and energy dispersive x-ray fluorescence spectrometry capability in a single device.
X-ray fluorescent spectroscopy (xe2x80x9cXRFxe2x80x9d) is typically conducted using a wave dispersive spectroscopy (xe2x80x9cWDSxe2x80x9d) or an energy dispersive spectroscopy (xe2x80x9cEDSxe2x80x9d) method. WDS utilizes Bragg diffraction and a precisely placed x-ray detector to determine the intensity of x-rays fluoresced from a sample as a function of wavelength. Because chemical elements have characteristic fluorescent spectra, the information derived from WDS can be used to perform quantitative analysis of the sample""s elemental content.
Conversely, EDS samples fluorescent x-rays from a sample without intervening diffraction, and measures x-ray flux as a function of energy. EDS x-ray detectors are typically less expensive than WDS detectors, but also provide lower resolution than WDS detectors. Further, EDS systems are typically less expensive to manufacture, providing a lower-cost alternative to WDS systems.
Because WDS and EDS systems have relative strengths and weaknesses, it is often desirable to have both systems available to perform a full range of XRF analysis on a sample. To somewhat reduce the expense and space requirements of doing so, hybrid WDS/EDS systems have previously been proposed.
For example, U.S. Pat. No. 5,978,442 to Kuwabara discloses an XRF system combining the elements of both WDS and EDS. That system includes a dispersing element used to disperse fluorescent x-rays toward a first x-ray detector, which is utilized in a WDS mode. By retracting the dispersing element from the fluorescent x-ray flux, the flux can be allowed to pass through to a second x-ray detector, which is suitable for and used to conduct EDS measurements.
Similarly, U.S. Pat. No. 4,959,848 to Parobek discloses a hybrid system for determining the thickness of a thin film and quantitative measurement of at least some of its elemental content. This system also involves the use of multiple detectors which are dedicated to either WDS or EDS measurements.
U.S. Pat. No. 6,285,734 to von Alfthan discloses a hybrid WDS/EDS system which requires multiple detectors, including individual monochromators, detectors, and measuring electronics for each element sought to be measured through WDS. Although this system can reduce the time for making measurements by eliminating the need to rotate the WDS x-ray detector through a range of angles, it also multiplies the equipment cost necessary to achieve the desired measurements.
Each of these systems attempts to provide both WDS and EDS measurements through a single apparatus. Such systems may reduce total requirements for laboratory space and utilities needed for operation, but they retain the expense of multiple detection systems, and each apparatus must be large enough to accommodate at least two sets of detectors with their appropriate hardware.
Thus, it is desirable to provide an apparatus capable of performing both WDS and EDS utilizing a single x-ray detector, thus reducing space requirements and overall cost of the apparatus. Further, such an apparatus can be efficiently switched from WDS mode to EDS mode or vice versa, providing great flexibility in utilizing the device to perform needed measurements.
As is typical in XRF spectroscopy, the sample from which measurements are taken is irradiated by an x-ray source, producing a flux of fluorescent x-rays from the sample""s surface. In WDS mode, the fluorescent x-ray flux is directed through a collimator and then to the surface of a rotatable crystal. For this invention, the rotatable crystal is preferably a multi-layer crystal such as OV-055A produced by Osmic, Inc. The fluorescent x-ray flux undergoes Bragg diffraction in the surface region of the rotatable crystal.
The invention also comprises an x-ray detector, mounted on a goniometer. In the preferred embodiment, the x-ray detector is a PIN diode detector such as PF1000 produced by Moxtek. Such a detector may be switched from a pulse mode to a cascade mode, allowing the detector to be used to perform WDS in pulse mode and EDS in cascade mode. This mode-switching capability increases the counter""s efficiency in each mode.
The goniometer provides controlled rotation of the x-ray detector about an arc. In WDS mode, the goniometer and the rotatable crystal are linked so that rotation of the rotatable crystal through an angle xcex8 will move the detector through an angle 2xcex8, thus maintaining the required positional relationship for measurements resulting from Bragg diffraction.
The goniometer is also aligned so that positioning at one of its endpoints of rotation positions the x-ray detector collinear with the fluorescent x-ray flux from the sample. In this position, the surface of the rotatable crystal is parallel to the fluorescent x-ray flux, so that the rotatable crystal no longer interacts with the x-ray flux. In this position, the detector is preferably switched to cascade mode for the purpose of performing EDS. To provide sufficient x-ray flux during EDS operations, it is preferred that the collimator is retracted from the x-ray flux, either by linkage to the goniometer or by another mechanism which provides for movement of the collimator.
To perform WDS, the x-ray detector is switched to pulse mode and rotated by the goniometer out of the EDS position. The linkage between the goniometer and the rotatable crystal will simultaneously bring the rotatable crystal into position so that Bragg diffraction occurs. In WDS mode, the collimator is inserted into the fluorescent x-ray flux.
It is also preferred that the total 2xcex8 range of the goniometer is restricted to approximately 50xc2x0. Although goniometers with greater range are available and although this limit may preclude exhaustive WDS analysis, this range of motion will provide adequate implementation of WDS for most purposes and allows the overall size of the apparatus to remain within desired limits.